Officials suspect Mr. Awlaki was a motivating force behind the shooting rampage in November by Maj. Nidal Hasan at Fort Hood, Texas. The attack left 13 US service members dead.

Awlaki is also believed to have played a role in preparing Nigerian Umar Abdulmutallab for his attempt to blow up a jetliner near Detroit on Christmas Day. Last week, Awlaki was named by the US Treasury Department as a “specially designed global terrorist.”

While these actions and investigations suggest US officials remain vigilant against home-grown threats from militant Islam, they do not address a more fundamental question: What would cause a life-long American to apparently choose the radical Islamic side in the US battle against Al Qaeda?

Mr. Rockwell’s plea agreement says that while residing in Virginia he converted to Islam and became a “strict adherent to the violent Jihad-promoting ideology of cleric [Awlaki].”

The document adds: “This included a personal conviction that it was [Rockwood’s] religious responsibility to exact revenge by death on anyone who desecrated Islam.”

Even after he moved to King Salmon, Alaska, Rockwell still followed Awlaki’s ideology, “including devotion to [Awlaki’s] violence-promoting works, 'Constants on the Path to Jihad' and '44 Ways to Jihad.' "

According to federal court documents filed in the Virginia case, Mr. Chesser told a federal agent that he first became interested in Islam in July 2008. His conversion came via videos, discussions, and debates broadcast on the Internet.

He is said to have found Awlaki particularly persuasive. Chesser sent several e-mail messages to Awlaki, and received two replies, according to federal documents.

In one e-mail, Chesser reportedly asked Awlaki to interpret dreams he’d had. In explaining one dream, Chesser reportedly told Awlaki that he prayed to Allah to let him join Al Shabab.